Pseudo-extension of frequency bands



vvvvvvvvv March 39, 194-3. L. A. DE ROSA 2,315,249

PSEUDQ-EXTENSION OF FREQU-FNUY BANDS Filed Oct. 8, 1941 5 Sheets-Sheet lM MWMMM v A p/ Nude [men for pf Fig.7

' March 30, 1943 DE ROSA 2,315,249

P SEUDO-EXTENSION OF FREQUENCY BANDS Filed Oct. 8, 1941 5 Sheets-Sheet 2March 30,1943; L, A, DE FmA 2,315,249

PSEUDO-EXTENSION OF FREQUENCY BANDS Filed 001;.- 8, 1941 5 Sheets-SheeiSMarch 3'0, 1943. DE ROSA 2,315,249

PSEUDO-EXTENSION OF FREQUENCY BANDS Filed Oct. 8, 1941 5 Sheets-Sheet 4K INVENTOR March 30, 1943. 1 5 RQSA 2,315,249

PSEUDO-EXTENSION OF FREQUENCY BANDS Filed Oct. 8, 1941 5 Sheets-Sheet 5INVENTOR Patented Mar. 30, 19 43 2,315,249 7 n PSEUDO-EXTENSIONOFVFREQUENCY BANDS Louis A. De Rosa, Dayton; out, Application October 81941, Serial No. 414, 137

13 Claims. (01. 179-4) v This invention relates to communication systemsand more precisely to an improved system of pseudo-extension.Pseudo-extension" inv cludes, for the purposes of this invention, themethods and means wherein an audio signal, at 6 some point or at sometime in its transmission either directly or indirectly to the ear, ismodified so that, while all the composite frequencies present in theoriginal. audio signal are not present in the signal ultimatelytransmitted to the .10

ear, the auditory perception is of a sound which has substantially allthe sonant characteristics of the original signal,

A pseudo-extension circuit is defined as an electrical circuit'for thetransmission of the lntelligence contained in a wide band of audiofrequencies through a narrow band transmission channel; wherein thesignal transmitted through the narrow band transmission channel isdistorted by the addition to it of extraneous modulation andinter-modulation frequencies lying within the frequency range of saidnarrow band transmission channel.

One object of this invention is to provide a method and indicate meanswhereby signals which are modified and transmitted by apseudo-extension" circuit are improved in quality and rendered morepleasing to the listener.

Another object of this invention is to lessen unwanted and extraneouspercussive and rasping sounds introduced in a signal by the"pseudoextension" process of the present art.

Another object of this invention is to reduce the apparent auditorydistortion resulting when 5 a transmitted'signal of transient nature ismodified by theaction of a non-linear transducer.

Another object of this invention is to reduce the apparent auditorydistortionresulting from the inter-modulation of at least two signals atleast one of which is transient in nature.

Further objects will be apparent from the description to be given.

Figs. 1 to.'7 show a single frequency signal of a transient nature andthe approximate frequency spectrums existent at various stages of thesignal.

Fig. 8 shows a diagrammatic circuit of one embodiment of the inventionas incorporated in a pseudo-extension non-linear circuit. v

Fig. 9 shows, diagrammatically,- an alternate circuit which may beemployed to utilize the advantages of the invention.

Fig. 10 shows, schematically, a simplified ciracter.

cuit which may also be used to-produce distor tionlesspseudo-extension." a t Fig. 11 illustrates a circuit including afrequency selective feed-back circuit-used to accomplish the object ofthis invention.

Fig. 12 illustrates an embodiment of this in-' vention 'to retain oreven accentuate the percussive quality of'a signal. 0

Fig; 13 shows, in'block diagram form, the use] of the circuits of Figs.8 to '12;.with associated a apparatus in an improved pseudo-extension"system.

Throughout the drawings,- parts performing;

like functions have been given the same reference characters andnumbers. 1 1

In order to clarifyand facilitate the understanding of this invention,it is believed'that an analysis ofan elementary form of a signal wave oftransient character would be serviceable. Accordingly, Fig. 1 shows sucha wave. 3

In Fig. 1 is shown an arbitrary single frequencysignal wave. having atransient char- A single frequency is shown for con I venience since bysuper-position of a plurality of frequencies',"signals of the typgenerally encountered infthe transmission; of audible sounds can'beobtained. r

This signal may be regarded as a single frequency only insofar asfrequency is measured by the reciprocal of the time between adjacentpeaks of the same, polarity. Actually every change in amplitudehasassociated with it an infinite number of side bandaprovided that theintervals between amplitude changes are infinitely long. While this isthe idealizedcase, nevertheless the side-bands encountered in practicalcases may extend to the tenth harmonic of the fundamental frequencyparticularly in sharplyintroduced low frequency waves.

If the wave had aninflnite interval of steadystate condition betweenamplitude variations then the side-bands would contain all frequenciesor would be a continuous spectrum; In other words, as the steady stateinterval approached infinity, the prime factor ofthe sidebandfrequencies would approach zero as a limit. An analysis of a practicalcase may be made by assuming. that the single frequency is amplitudemodulated by a frequency modulated signal, wherethe frequency modulationis the variations in the intervals between successivechanges inamplitude. Therefore, in the practicalgcase the prime factor of theside-band'frequencies-is the reciprocal of the time betweenthe longestin- 2 Y l I 2,315,249

tervals during which there is no amplitude change.

As the rate of increase or decrease of the amplitude of a wave such asthat shown in Fig. 1 becomes slower and slower; the mean energy of theassociated transients is shifted towards the lower frequencies.Simultaneously, the amplitude of the. side-bands becomes less and less.until for steady-state conditions, the side-bands become zero.

Returningto Fig. 1, the wave in starting from a zero reference time inits build-up would, at some time t1 have a frequency spectrum associatedwith it similar to that shown in Fig. 2.

The side-bands are. grouped about the steady state frequency Qf'the waveand their amplitudes are an inverse function of their displacement fromfrequency f.

At t: of Fig. 1, when the amplitude of the wave has almost reached asteady state value, the

component frequencies have a spectrum similar to Fig, 3. It is to benoted that the side-bands have diminished in amplitude so that onlythose side-band frequencies in the immediate neighborhood of thesteady-state frequency I have important amplitudes. r

At time t: the illustrative wave is again changed in amplitude. Thefrequency spectrum consequently, due to this transient, becomes morecomplex and theifrequencies more distantly removed from-the steady statefrequency 1- again become prominent. The spectrum assumes therefore, acomplexity such as is shown in Fig. 4.

At time t4 when the rate of increase of the envelope of the amplitudeshas almost reached a steady-state condition, the magnitude of theside-bands has again'diminished and, as before, only the frequencies inthe neighborhood of the steady-state frequency f have importantamplitudes. The frequency spectrum at this time is shown in Fig. 5.

At time is after a relatively long interval the signal wave decreasessuddenly in amplitude and immediately the side-band frequencies againappear. The spectrum of the side-band frequencies for this condition isshown in Fig. 6.

At time is the amplitude of the'arbitrary wave of Fig. 1 has diminishedconsiderablyand is approaching zero. The side-bands are thereforelocalizing in the immediate vicinity of thesteadystate frequency 'f andas they localize, the amplitude of the frequency 1 drops off so that,when the wave of Fig.1 has reached a practical zero emanating from acello or bass viol having a split or cracked body.

When the high frequencies are pseudo-extended this deleterious effect isparticularly noticeable in speech when the fricative consonants arereproduced.

This invention describes definite methods and means for reducing thedistortion encountered in prior art pseudo-extension processes. Thecircuits describedhereinafter may also, with resulting improvement, beused in place of the nonlinear circuit elements of the pseudo-extensionsystem described in the applicant's co-pending U. S. application No.348,359, filed July 30, 1940.

In order to explain the operation of this invention, an analysis of whathappens when the arbitrary functionof Fig. 1 is applied on a nonlineartransducer, is advisable. Returning then,

to a study of Fig. 1, let it be assumed that the wave representedthereby is applied to a modu-' lator or non-linear element such as asquare law detectonand, further that the transmission band of thislatter device is also uniform between the frequencies 11 and In.

The resulting frequency spectrum at time h which appears in'the outputof the non-linear device is similar to that shown in Fig. 2 except that,due to the resulting inter-modulation products, the magnitudes of theside-bands are greatly increased; in other words, the spectrum issimilar to that obtained if the original wave had been building up morerapidly at that time. The effect on the listener, hence, is that of asignal building up more rapidly and sharply than the original signal ofFig. 1 warranted.

Proceeding to a later time t: it is noted that at :this time the signalhas practically reached a steady state value, hence the frequencycomponents at this time, as shown by Fig. 3 are grouped closely aboutthe steady-state frequency f. The output ofthe non-linear devicehowever, due to the inter-modulation frequencies introduced by itsaction,will produce a spectrum wherein the side-bands are greater inamplitude than those present in Fig. 3. That is, the spectrum resultingfrom the action of the non-linear device will at time t2 actually bemore nearly that shown by Fig. 2. Consequently, the ear, which iflistening to the original wave would hear pracmagnitude, the side-bandshave become merged into a position occupied by I which has in themeantime diminished and reached a zero magnitude.

Fig. 7 shows the spectrum analysis at time t1 when the wave amplitude isapproaching a practical zero value. 4 l

In general, prior art pseudo-extension" circuits include as an essentialelement, a non-linear transducer. This non-linear transducer may be asquare law detector, a class "0 amplifier tube. a combination of both,or any circuit having a non-linear input vs. outputcharacteristics.

These prior art circuits, while producing a measure of fpseudo-extensionhave associated with transients occurring in the input signal wave, anaugmented percussive quality and a 'raspy harsh effect. f

When the bas's'frequencies are "pseudo-extended the effect noticeable ontransients may tically no transient effect, due to the action of themodulator, would detect a sizeable transient effect.

Thus it is evident that the introduction of a.

non-linear device will produce an exaggerated transient quality whichpersistsafter the transient quality of the original signal has beenremoved.

The methods and means for producing a pseudo-extended signal free fromthis abnormal duration of transient quality, which to the ear results inthe presence of extraneous harshness and raspy effects, is the essenceof this invention,

linear device will contain higher amplitude sidebands than those shownby Fig.4, so that the resulting spectrum will be more nearly similar tothat ofFig. 2.

At time t4; the spectrum of the modulator output will be more nearlysimilar to that shown in .and is leveling out in amplitude thisinvention.

I off so that it operates as cuit load impedance R4 andC4.

1 C4 has charged. There is measurable from the time plied thru thebattery E01,

Flg. 4; the amplitudes of the side-bands again having been increased bythe non-linear char- I acteristi'c.

At tlmett the output er the non-linear device a will have a frequencyspectrum in which the sidebands are of much greater amplitude thanindicated by Fig. 6.

At time is the transient V V gerated by the non-linear device, so thatthe transient quality is still detectable even when the signal hasapproached a' steady-state value as it approaches a zero value.

Fig. 8 shows, diagrammatically, one 'circuit which may be'used toaccomplish the object of The input signal is impressed across theterminal marked-Elna This signal, if the pseudoextension isto be appliedmainly to the bass frequencies; is preferably applied to the thru a lowpass filter F. A low' pass filter of one section prototype constant Ktype having a cutoff frequency of aboutlOO cycles has been foundsatisfactory. The output of the filter is divided into two paths, theone to the left and consisting of C1; R1; R2; T1; R and C5 being thenon-- linear circuit; andthe, lower path, consisting of CajRa; R1; C4;R5; andTz is the control circuit. The purpose of this control circuit isto. change the bias of the tube T1 so that T1 operates as a non-lineardevice only when the signal wave applied to its grid has .attained arelatively steady-state condition. A portion of the signalfrom theoutput of the filter F'is impressed, by means of the capacitor C3 andthe variable potentiometer R3, between gridof tube T2. Tube the batteryE02 to cuta class C amplifier.

the cathode and control T2 isbiased by means of indicatedon the dia-Thistime lag is a function ofithe product of R and C, for increasingsignals, and a function of the product R4 and C4 for decreasing inputsignals.

The point where, on the increasing amplitude envelope of theinput signalthe tube T2 begins to conduct can be adjusted by the setting of thepotentiometer R3. The rate at which the rectified signal across Ridecays when the input sigcircuit is supplied by the bat-' eifect isagain exaginput capacity of tube T1 may be made small in comparison tothe time constantso; R5 C4 and R4 C4 so that these latterconsta'ntswill'detern'iinev the build-up and decay respectively of thevoltage applied tothe gridci tube T1 from the plate circuit of tube T2.For a transient associated with an increase in amplitude, the opera tionof the circuit of Fig. 8 is such thatthe tube T1 operates as a lineardevice until the input signal hasreached a relatively steady-stateconditionwhere after the tube ,Troperatesas a nonlinear device andcontinues to do so until a dee cay transient occurs inthe input signalat which time the tube T1 again operates as a linear ampliher.

The output Eout of T1. taken across its plate load Re through theblocking condenser C5. is

therefore'fairly free from distortion during the transient intervals ofthe inputsignal, and the extraneous; unpleasant and foreign auditoryeffects are eliminated from the pseudo-extended" signal. This output maythen be used in place of the uncontrolled output of the non-lineardevice associated withvarious pseudo-extension circuits. I i

The time constants of the delay circuits must be chosen with a view tothe frequencies which are being impressed on the tube T1.Experimentally'it has been found-that satisfactory results areobtainable when the delay between the onset of the signal and theoperation of T1 as a nonlinear device is, as a function of the. meanfrequency applied to the grid-of ,tube T1, approximately 20 millisecondsfor 200 cycles per'second 40 milliseconds for 100 cycles per second;

80. milliseconds for cycles'pe'r second;

a 10 milliseconds for frequencies between 500 and 4000 cycles; 1 andabout 40 milliseconds for frequencies higher than 5000 cycles persecond. For decay transients, the tube T1 should preferably be returnedto its linear operation as soon" 'as the decay transients set in; anapproximate time being 8 to l0 milliseconds.

1 Another embodiment of this invention is shown in Fig. 9. This circuitaccomplishes the object of this invention in one possible alternatemanner. The signalto be pseudo-extended is ap- \plied to the input ofthefilter F. This filter may.

if the low audio frequencies are to be'operated upon, have the samecharacteristics as that described vin connection with the circuit ofFig. 8.

If the high frequencies are to be pseudo-extended then a singlestagehigh pass constant K pro totype having a cut-off fre'quency ofapproximately 2500 cycles has been found satisfactory. The output of thefilter f is applied to an electronic compressor circuit of the type wellknown to the art, consisting of tubes T3, T1 and the double diode T5.The operation of this compressor circuit is as follows. The signal fromthe output nal decreases is adjustable byvarying either the value of C4or the value of R4. I

The rectified signal appearing across R4 is apof .the tube T1.

absence. of

a voltage across R4. is essentially a non-distorting linear amplifier. Apositivevoltage appearing across R4 will destroy the bias of tube Trandcause it to operate as a grid leak detector or square law non-linearamplifier. The delay time of the networks R1 01 and R2 C2 where C2 isthe R1 and R2 tothe grid. 4 The battery Em is selected so thatthe'operation of tube T1 in the of the filter F isapplied by means ofthe'condenser Cs and the grid resistor R1 to one control grid of a twocontrol grid pentode T3. The amplified signal appearing in the platecircuit of Ta is impressed by means of the condenser C7 to the level ofthe signal at the grid of T4 being adjustable by means of thepotentiometer R3. 'Theamplified signal appearingin theplate circuit of Tis applied through the audio transformer Tm to a double diode rectifierT5. A rectifiedsignal appears across the-diode load-resistance R1, therate at which it builds up being determined by the product of'the seriesre- 4 sistor Ru and the capacitor C4. This rectified voltage iss'ppliedto the second control grid of Ta and being a negative voltage,decreases the transconductance of ".thetube Ts resulting in a decreasein series, between one terminal of the secondary f the transformer andthe control grid of tubeTi.

when the signal applied to the grid of tube T1 is great'erthan its biaspotential, grid current will: flow 1 through the resistance R: causingthe transconductance of the tube to decrease. The tube will consequentlyact as a non-linear device. Bvnsing a tube fOrT'a which has a remotecutof! characteristic for the grid to which the diode rectifled'yoltageis applied, the compressor circuit can bemade'tofsuppress transientsabove a certain amplitude of input signal.

The bias of :tube T1-is adjusted so that tube T1 acts as a non-lineardevice when the amplitude of the input signal to the compressor circuithas reached an amplitude beyond which further-increases are minimized.

The selection of the time constants associated with'the diode loadshould be chosen so that the ripple voltage appearing across the diodeload and applied toits associated grid of T: is approximately tenper'cent of the signal voltage applied to the other grid of Ta. Thiscondition holds for the charging time of C4 through Rs andthe associatedclosed circuit and for discharg'etime of 04 as determined by the valueof R4, and-the equivalent shunt impedance across C4 due to. theassociated shunt meshes.

The output voltage limit is taken from the plate circuit load resistorR0 through the blocking condenser Cs.

The circuits of Fig. 8 and Fig. 9 show twoembodiments of the invention,and in addition they illustrate two difierent solutions to the problemof the elimination of transient distortion in a nonlinear device. f

The first circuit, that of Fig. 8 varies the linearity of the non-lineardevice so thatit operates essentially as a class A amplifier during thetime that a transient is impressed on its 1nput.

The second circuit arrangement, that .of 1"ig. 9, is illustrative ofthat class of circuits within the scope of this invention wherein thetransients are minimized at those times when the non-linear deviceoperates as such.

In the circuit of Fig. 10 is shown another possible alternatearrangement to produce the object of this invention.

If a signal is suddenly applied to a transducer having a narrowacceptance band, such as a tuned resonant circuit, the build-up ofthe,sig-.

nal in the output of the transducer is very slow. In other words, asharply increasing or decreasing amplitude will not be effective as suchin the output of a transmission network having a narrow frequencyacceptance band.

vThis property of a tuned circuit may be used to prevent sharponset anddecay transients with their associated side-bands from producing, in theoutput of a non-linear device, the. characteristic transient distortionoi the prior art pseudo-extendedsignals. a

In Fig.. 10 the signal to'be impressed on the This tube is arranged witha i non-linear device of a "pseudo-extension circuit is designated atE111. This signal is divided'into two channels, the first of these isapplied through the volume control. R1 to one control grid of amixertube-such as the present day 6L7, desig-' The other channel isvapplied to a fitted as T13, tuned circuit Tc tuned to the region of thefrequencies which are to be pseudo-extended. When the high frequencyspectrum of a signal is to be transmitted over a transmission circuithaving a low :pass characteristic extending only to say, 4000 cycles,satisfactory results have been obtained with a resonance circuit for Totuned to about 6000 cycles. of a signal is to be transmitted over acircuit having a high pass characteristic extending to, for example 120cycles, the tuned circuit Tc should betuned to the neighborhoodofcycles.

The narrow band output of the tuned circuit To is amplified by ,a linearamplifier designated operate, by virtue ofthe initial bias, as a classAamplifier for small signals applied'to its grid. The volume control Ruleadjusted so that the signals appearing at the grid of T1 via the firstchannel, that is. impressed on the first control grid. 01 of Tn, are ofsufliciently low magnitude so as to be amplified by T1 withoutdistortion.

The amplifier "A" contained in the second channel. namely that channelimpressed on the second control grid G: of tube T13, causes an amplifiedsecond channel signal to be impressed, via T1: on the control grid oftube. T1. This signal, by circuit design. is made approximately tentimes the magnitude 'of the, first channel signal impressed on the gridof T1. The tube T1 therefore. acts as a non-linear device in thepresence of a signal from the second channel.

The signal appearing across the plate load resistor R1: of tube T1 isutilized, through condenser Cs, as the output of the non-linear circuitin prior art pseudoextension" circuits.

The circuit of Fig. 10 described above and the circuitof Fig. 11 to bedescribed operate to prefrom the harsh, rasping, spuriously fricativesoundsassociated with prior art pseudo-ex tended signals.

Fig. 11 illustrates a circuit which may be used to realize the utilityof this invention. Basically, its operationis similar to that of Fig. 10but it is shown in order to indicate one of a large number ofmodifications which are within the scope of If the low frequencyspectrum this invention, and whichutilize the method and means describedherein. I

Referring to Fig. 11, Ein'is an audio signal containing the frequencieswhich are to be pseudoextended. This signal Em is applied to one grid G1of a two control grid pentode T14 of the present day 6 L7 type. R1 isthe grid resistance to maintain the bias on grid G1. The tube is biasedso as to operate as a linear amplifier by the conventional self-biasingby-passed cathode resistor arrangement as indicated in'the figure.

An amplified signalappears across the plate load resistor R20 of tubeT14. Those frequencies appearing in this amplified signal which are tobe pseudo-extended are fed-back through the blocking condenser C11 andthe network R23, C13, R22, C12 to the second grid G2 of the tube T14.The network 1223,013, R22, 012 is selected so as to produce anapproximate phase shift-of 180,degrees for these frequencies.

The potentiometer is used to control the voltage of the fed-back signalso as to prevent oscillation. The phase .of this fedback signal beingsuch that regeneration will occur for frequencies to be pseudo-extended,the tube T14 will therefore selectively amplify these frequencies to amuch greater extent thanother frequencies present in the signal Em.'Effectively therefore the tube T1; acts as a tuned amplifier having'amaximum gain in the neighborhood of the frequencies to bepseudo-extended. The principle of operation of the circuit of Fig. andthat of Fig. 11, is similar, that is, in Fig. 11, due to the relativelysharp frequency characteristic of the tube T14 and its associatedfeed-back circuit, the side-bands present during changes in amplitude ofthe preferentially amplified signal are limited in amplitude. Limitingthe number and amplitude of these side-bands produces; after the thugrestricted signal is acted upon by the modulator or non-linear amplifiertube T1, a signal wherein the intermodulation products due to theseside-band frequencies are. reduced in number and in amplitude.Therefore, the resulting signal produces an apparently undistortedeffect upon the listener.

The signal appearing across the plate load resistor R is impressed,through the blocking condenser C11 to the potentiometer R1. The settingof the potentiometer is adjusted so that. in absence of a regeneratedsignal across its terminals, the ordinarily amplified voltage isinsuflicient to drive the tube T1 oil." the linear portion of its gridvoltage-plate current characteristic as determined by the grid biasproduced by the oath- Fig. 12 shows, diagrammatically an arrangement ofa circuit utilizing the novelty of this invention which may be used toretain the or even augment the transient quality of asignal and yetavoid the "harsh and fricative sounds due tothe prolongment ofthetransient stateof a signal due to the action of anon-linear trans-'ducer. In Fig. 12, the signal Em to be pseudoextended is impressed onthe input of thefilter section This filter section may be a sin-' 1 vglestage constant K prototype or its'equivalent.

If the circuit is to be used to pseudo-extend the high frequencyspectrum of. thegsignal Em,

- the filter F is arranged as a. high pass, a representative cut-offfrequency being 3000" cycles. If the circuit is to operate for. basefrequencies, the filterF may be a low pass having -a cut-off frequencyof about 120 cycles.

The output of the filter Fin either case is impressed on the grid of a.linear amplifier tube Ts, R1 being the conventional grid resistor as-.

, sociated with the grid circuitof T6. The amplifled signal appearing intheplate circuit of tube To is divided into three channels consisting oftwo control circuits and one main circuit.

The main circuit, by means of the transformer TR; having twosecondarywindings, is further divided into two secondary circuits. The first ofvoltage vs. platecurrent characteristic of the the linear or class. Aamplifier'tube T1 appears ode resistor R24 bypassed by the condenserC14.

When a regenerated signal is present however, the tube T1, dueto themuch larger voltage of this signaL'will, and aided in this direction bythe grid'resistor' Rz, act as a non-linear device and produceinter-modulation and harmonic 7 components in its plate circuit. Thesignal appearing across the plate load R18 is utilizable in place of theoutput of the non-linear'device of the related prior art circuits.

In certain audio transmission systems it is often desired topseudo-extend the audio signal but also transmit, together with theprocessed signal, the full percussive and transient quality of theoriginal signal. The suppression of the transients associated with theoriginal signal before processing, results in the loss of a certainamount of articulation in speech and the. loss of clangor and percussiongenerally associated with some sounds.

tube T12.

When the input signal Em is suddenly increased in amplitude, that is,becomes transient in nature, the amplified portion of this signal whichis impressed through the blocking condenser and the potentiometer Ra tothe grid of as an amplified signal in the plate circuit of T7. Thisplate circuit signal is coupled by means of the audio transformer 'I'Reto a' full wave diode rectifying tube T8. The rapidity with which theresulting rectified voltage appears across the diode load R9, R1o and C9is determined by the charging time of the condenser C2 through theseries resistor R9. Foran instant, therefore upon the sudden onset of atransient signalno voltage appears across the resistor Rio. Thepotential of the junctionof Ru and R10 is therefore during this instant,the same as the potential of theground or.common connector. As thecondenser C9 charges, the potential at the junction of R9 and R10becomes increasingly negative with respect to the common connector andtherefore also withrespect to the cathode of the tube T12. Y

The tube Tiahaving no grid bias except that available from the junctionof R and R10 through the lower secondary of the transformer TRa, willtherefore, during thefirst instant of the sudden onset of the inputsignal Em, operate as a non linear device. The augmented transientsidebands due to the inter-modulation frequencies are introduced by thetube T12 during the interval wherein its grid has a zero bias withrespect to its cathode. The next instant after the onset of the signal,the negative voltage [appearing at the junction of R9 and R10 andapplied through R13 to the grid of T12 establishes a negative bias onthis grid and causes the tube T12 to act as a linear or class "A"amplifier. The signal appearing in the plate circuit of the tube T12 isthen applied to the output through the audio transformer TRs.

The upper control circuit consisting of R11, T9, TR4 and T10 controlsthe grid bias of the tube T11. The tube'Tn is biased by the battery Ectso as to operate, in the absence of a voltage across the load resistorR15 of the rectifier T10, as a linear or class A" amplifier. When theinput signal Em is suddenly applied, it is amplified by the tube T6 andthat portion of it appearing on the grid of T9 is amplified further,and, by means of the audio transformer TR4, is applied to the diode T10through the rectifier load impedance R14, C10 and R15. The condenser C10charges slowly through the limiting resistor R14 and as it charges, thepotential of the cathode of the rectifier T10 gradually rises withrespect to ground. The cathode is connected through the battery E04, theupper secondary winding of the transformer TR: and R12, to the grid ofthe tube T11. Consequently as the cathode 01 T10 rises in potential in apositive direction, the voltage of the battery E} is graduallycounteracted and the negative bias of the tube T11 is graduallydestroyed. As the bias of T11 is destroyed the grid operates with toolittle bias and finally without bias so that the tube T11 will operateas a nonlinear device similar to a grid leak detector. R12,

a resistance in the grid circuit provides an additional voltage dropacross it when, on the positive halves of the main channel signal, gridcurrent flows in the grid circuit or the tube T11. This action biasesthe grid more heavily at these times and accentuates the non-linearity.

The signal appearing in'the plate circuits of tubes T11 and T12 isimpressed across the upper.

Apower amplifier with an associated filter is connected across thesecondary winding of TRs. The characteristics of the filter shouldapproximate the frequency characteristic of the limited i'requencyacceptance band of the apparatus through which the pseudo-extendedsignal is to be transmitted. To the output terminals of the poweramplifier and filter is connected a loud speaker. This auxiliaryapparatus is shown on rug.- 12 by the dotted lines, PA being the poweramplifier, Fx beingthe filter and LS being the loud speaker. Thevertical plates of a cathoderay oscilloscope are connected between theplate of T11 and ground.

A monotone signal whose frequency lies approximately midway in thefrequency band of the signal which is to be pseudo-extended is abruptlyapplied and removed intermittently to the input Em of the circuit ofFig. 12. The setting of Ru and the value of the variable resistance R14are adjusted simultaneously to a point where while the cathode rayoscilloscope deflection appears to be unilateral, sharp clicks are notheard in the loud speaker upon the application and removal of the inputsignal.

After this is done, the shorting lead between the grid of T1: and itscathode is removed and the setting of R8 and R9 adjusted so that a clickis evident in the loud speaker upon the application and removal of theinput signal. Re and R9 are adjusted so that the clicks thus introducedshould be comparable in quality and duration to those which are obtainedwhen the intermittent signal is applied directly to a loud speakerhaving a wide band response.

As heretofore stated and described, this invention provides methods andmeans for producing a pseudo-extended signal which is relativelyexaggerated and prolonged transient quality apparent as distortion,these unpleasant sounds suddenly applied tothe input (Em) an ampli-' Ified signal containing a plurality of side-bands appears in the platecircuit of T12. (Also, during the first instant, an amplifiedundistorted signal appears in the plate circuit of T11. These signalsare combined in the transformer TR: and are available across the outputterminals of TRs. Gradually, after the first instant, the plate circuitsignal of T1: becomes undistorted, as a negative potential builds upacross R10 and Co; at which time the plate circuit signal of T11 isstill relatively undistorted sincethe condenser Cm is still only.partially charged. The signal Emit is therefore relatively undistortedfor an interval after the first, after which, when C10 has chargedsufficiently to cause the voltage across it to counteract the initialgrid bias of T11, the plate circuit signal of T11 becomes distortedproducing intermodulation between harmonic components of those.frequencies impressed on its grid.

The plurality of elements involved in the circuit of Fig. 12 make itinadvisable to-fix precise circuit constants for its operation. The timeconstants of the various complicated and interconnected meshes wouldrender the synthesis of the circuit extremely laborious and difllcult.Accordingly, a simple method of adjustment has been evolved.

being present to a noticeable degree in prior art pseudo-extendedsignals. In other words, by the use of the circuits herein described, asound having a wide audible frequency spectrum may be transmitted to alistener over a transmission link having a narrow frequency passcharacteristic, and while the sound reaching the listener's ears doesnot contain a wide band of audible frequencies, nevertheless, the effectis substantially the same as though all the audible frequenciescontained in the original sound were available, without distortion, tothe listener.

To produce this pseudo-extension effect obviously the representativecircuits of Figs. 8 to 12 must be used in conjunction with a soundsource, microphone, phonograph record or other primary or secondarysources of audio signals and necessary signal levels produced with theaid of amplifier or attenuators, active and passive transducers, all ofwhich are well known to the art.

In order to indicate more clearly the auxiliary circuits and apparatusnecessary in the improved pseudo-extension system of this invention therepresentative diagram of Fig. 13 is shown.

Fig. 13 shows, in block diagram form, since the elements thus shown arewell known to the art, the utilization of this invention to one of manypossible and varied applications. The system of Fig. 13 shows the use ofthemethod and means oithis invention to the transmission of sounds by aradio system. S is a source of audible sound as for example anorchestra. The sounds emanating from this source are intercepted by themicrophone and after conversion to an elec- I area 7 m,

lathe to emphasizethepoint that the input filter F-of Figs. 8, 9' and 12is not necessarily an indis- ;the improved fpseudoextensionil systemREC,

subject of this invention. The output of theam. plifier AMP] isconsidered as the' input Em of the 1 representative circuits. of Figs.8., to 12,shown herewith. After. being ,modified by, the. circuit meansof: this invention; the resulting output signal Eout 'is impressedand'used to modulate a radio frequency carrier; in the usual manner inthe radio transmitter'XT. The portion-ofthe transmission link havingarestricted frequency pass characteristic maybe anywhere in the entiretransmission link beyond theoutputof the circuit PEG, and m'ayeven bedueto a;hearing' deficiency in the ultimate listener.'- If. there aretricahaudio eal are am lified byvthe amplifier =The output of thisamplifier isiapplied to,

pensable feature or the circuits shown herewith.

The .class 10f circuits shown: in Figs. 8 to 12 may be used similarly tothoserecogni' zed as prior art circuits, yie lding;in the place of thelatter an improved and undistorted result. Obviously, therefore,thesecircuits and the principles gov erning their. operation can be usedin conjunction with: the:;consonator {-circuit described in myco-existing application no frequency limitations in the transmissionlink f but restricted,side-bandradio transmission is desired,ythe,filterFm of Fig. 13 may be interposed 'in the transmittercircuit.

In applications :where the magnitude of the original soundsourcefallingtwithin the trans- I mission-bandis not suflicient to produce abal- :anced or realistic signal to the listener, apart of v.Rlllof Fig.13, without requiring circuit modifications in that receiver.

The processor pseudo-extension need not be limited so as toobtain asignal containing as an example frequencies from, 120 cycles tov 10,000cycles per second andbetween'which limits are included theintermodulation and harmonic-produ g evident to-persons familiar with.the pertinent art, without depart- Other embodimentswill be ing fromthespirit or scope of my invention. As

an example, an equivalent method. might be used,

and the results be substantiallythe same,- if the circuits were made tooperate ona carrier frequencymodulated.bythe audio signal to beipseudo-extended,in which case whilethe operations would be performed ata difierent range I of frequencies, the method and means would besubstantially the same as indicated herein. The

illustrations included herewith and the descripa I tions included hereinshould serve, in view of the 'WhatIclaimis: i v d l 1. In a fps eudoextension.circuit including a entire disclosure, to indicate rather.than limit the'generaldesign, and to illustrate rather than 0 specifythe components and elements ofthe various representative circuits sincemany modifications may be, made ;without departing from the scope of myinvention as set. forthin the appended claims.

non-linear device and; a source of audio fre- I the non-linear device,means for varying the non- ,linearity of said non-linear device duringchanges ucts .of frequencies lower than 120 cycles; nor

, need it be limited to producing a signal contain ing, as a furtherexample, frequenciesfrom 30 to 4000 cycles per second with theintermodulation products of frequencies, higher than 4000 cycles persecond. For, by operating onthe transient portions of the signals as.described above,'a.ddi-

- tional intelligence and a more pleasing signal can be produced byoperating on any or all of the frequencies associated with a signal.More specifically, the middle register of a signal, that is, thosefrequencies in a signal fallingwithin the narrow acceptance band of atransmission link may be operated upon bythe proceduredisclosed hereinand a more pleasant and'more quency modulation meansior .impressing saidaudiofrequency modulations upon the inputof in the amplitude of theimpressed signal, means for collecting at leasta part of the-output ofsaid non-linear device,- and means fortransmitting the collected signal.a

2. In a fpseudo-extension circuit including. a:

non-linear device and a source of audio frequency modulations, means fordividing the audio frequency modulations into two channels, means forselecting a part of the frequencies contained in the first of-these twochannels, means for realistic signal will result. When these latterfrequencies are operated upon, the, resulting signal isgiven moresonority and fullness, similar to'an eifect produceable by increasingthe amplitude of a sound to loudinten'sitles.

In addition, the introduction of both the highest'and lowest frequenciessimultaneously into the pseudo-extension circuit as described here- Itofore will result in an increase of quality and articulation conveyedby the resulting signal when transmitted through a system whoseacceptance band extends only through a middle range of frequencies.Thisis so since the intermodulation of the highest-with the lowestfrequencies will result in inter-modulation products outsidethe-acceptance band ofthe subsequent transmission circuit.

. Since this invention can be usedto operate on.

a wide range of input signal frequencies, the

input filters which determine the range of signal frequencies to bepseudo-extended" have, in

Figs. 10 and 11, been omitted. This has been impressing said selectedpart of the first channel on the input of the non-linear device, meansfor varying the non-linearity of said non-linear device during changesin amplitude in' the input signal impressed on" said non-linear device,

fmeans for combining the output signal of said non-linear device withthe signal of the second of the two channels, and means for transmittingat least apart of the combined signal.

3. A method of increasing-the apparent frequency range of a transmittedaudio signal comprising the steps of selecting at least apart of theaudio signal modulations, limiting thechanges in amplitude envelope ofthe selected signal by the action.of a compressor circuit so as toproduce substantially a constant amplitude frequency distortionlesssignal, impressing the thus limited audio signal on the input of anonlinear device, and collecting and transmitting at leasta part oitheoutput of said non-linear device.

4. A method; of increasing the apparent free I quency range of atransmitted audio signal comprising the steps of: selecting at least apart of the audio signal modulations, limiting the' changes in amplitudeof the selected signal by the action of a compressor circuit, impressingthe together with the collected and transmitted part of the output ofthe non-linear device. 1

6. Amethod of increasing the apparentfrequency range of a transmittedaudio signal according to claim 4, in which an'unmodifled part of theinput signal is combined and transmitted together with the collected andtransmitted part of the output'of the varied non-linear device.'

7. A method of increasing theapparent frequency range of a transmittedaudio signal comprising the steps of selecting at least a part of theaudio signal modulations, increasing the relative amplitude of a portionof the frequenciescontained in said selectedpart, impressing thedistorted signal thus resulting on the'input 01. a non-linear device,and collecting and transmitting at least a part of the output signal ofsaid non-linear device.

8. A method of increasing the apparent frequency range of a transmittedaudio signal comprising the steps of: selecting at least a part of theaudio signal modulations, increasing the relative amplitude of a portionof the frequencies contained in said selected part, impressing theresulting distorted signalon the input of a nonlinear device, varyingthe degree of non-linearity of said non-linear device during changes inamplitude of the original audio signal, and collecting and transmittingat least a part of the output signal of the varied non-linear device.

9.' A method of increasing the apparent Irequency range of a transmittedaudio-signal according to claim 7, in which at least a part of theunmodified original input signal is combined and transmitted togetherwith the collected and transmitting part of the non-linear device.

10. A method of increasing the apparent frequency range of a transmittedaudio signal comprising the steps of: dividing the audio signal into twochannels, impressing the first of these.

two channels on the input of a non-linear device, varying thenon-linearity of the non-linear device during'the initial time of achange in amplitudeof the signal of the first channel, impressing thesignal of the second of the two channelson the input of a secondnon-linear device, varying the non-linearity of this second non-lineardevice. during a change in amplitude of the signal of the secondchannel, collecting at least a part of the signals appearing in theoutputs of both nonlinear devices, and combining and transmitting saidcollected parts,

11. A method 'of increasing the app rent frequency range of atransmitted audio signal according to claim 8, in which an unmodifiedpart of the original signal is combined and transmitted together withthe collected'and transmitted parts of the outputs of the non-lineardevices.

12. In a system of the class described, the combination with anon-linear device comprising means for varying the non-linearcharacteristic of said non-linear device during the times whenthe signalimpre'ssed'on the input oi'said non-linear device is undergoing a changein its amplitude envelope.

13. In a system of the class described, the combination with anon-lineardevice comprising means for impressing an audio signal uponthe input of said non-linear device, means for vary- LOUIS A. Di: ROSA.

